In order to be normally turned on, an N-ch MOS switching device that applies energization control to a multiphase motor requires a gate voltage the same as or a predetermined value higher than the source voltage.
In this situation, a charge pump circuit is known; in order to obtain a high voltage, the charge pump circuit steps up a power-source voltage so as to obtain a stepped-up voltage.
In an electronic device mounted in a vehicle, it is required to suppress electromagnetic noise such as radio noise; in general, radio noise is produced at a timing of a change, in a current consumption, that is caused when a semiconductor switch is turned on or off, and the noise level is liable to rise in proportion to the steepness of a current change.
FIG. 3 illustrates the configuration of an ordinary charge pump circuit; when a switch (SW3) 3 is turned off and a switch (SW4) 4 is turned on, a pump capacitor (C2) 12 is charged with a current supplied from a B terminal of a source power supply via a diode (D1) 7; when the switch (SW3) 3 is turned on and the switch (SW4) 4 is turned off, the electric charge stored in the pump capacitor (C2) 12 is discharged to a tank capacitor (CU) 13 via the diode (D2) 8.
Because due to the operation of the charge pump circuit, the current from the B terminal steeply changes, the level of radio noise rises.
For example, Patent Document 1 discloses an invention in which a filter is provided between a charge pump circuit and a power source so that switching noise is eliminated.